In recent years, in response to the emerging energy resource problems and global environmental problems, the photovoltaic power generation has increased in importance. Thus, expansion of use of the photovoltaic power generation and development of an energy supply technology have been carried out enthusiastically.
The photovoltaic power generation aims at a general-purpose power-level generating cost from the viewpoint of the cost efficiency. This leads to a demand for a solar cell capable of reducing the module manufacturing cost, and having a high generating efficiency.
As a method for enhancing the generating efficiency, an increase in focusing efficiency to a photovoltaic cell can be considered. Conventionally, as optical members for concentrating light, mention may be made of (1) lenses (spherical and aspherical types), (2) concave mirrors, (3) planar films, and the like.
As the planar films, optical members such as a Fresnel lens have been under development (see, e.g., Patent Document 1). However, conventional planar films each cannot concentrate light incident at a shallow angle with respect to the in-plane direction of the planar film. When used as a light-concentrating film for solar cell, in order to perform electric power generation with efficiency, the planar film is required to be a so-called tracking type solar cell in which the orientation of the solar battery cell is allowed to track the direction of radiation of sunlight. The tracking type solar cell is a system controlled so as to cause a focusing element to continuously face the direction of the sun. Accordingly, the whole system is in a complicated structure, posing problems in terms of cost and facilities.
Further, as the planar films, optical elements using hologram are also known. For example, mention may be made of a hologram manufactured by laser imprinting a hologram diffraction pattern onto a photosensitive material. For the formation of such a hologram diffraction pattern, there is used the following method: for a photosensitive material such as gelatin dichromate, the interference fringes of a reference light and an object light such as a laser light are recorded on the photosensitive material surface, and information of the complex amplitude of the object light is optically fixed on the recording surface. However, the photosensitive material lacks the durability (moisture resistance, weather resistance). For this reason, when the planar film having the hologram diffraction pattern is used for a solar cell, it is to be mounted in the inside of the solar battery cell. This results in a solar battery cell of the total internal reflection system as disclosed in, for example, Non-Patent Document 1. Therefore, the solar cell does not concentrate light in a wide angular range (omnidirectional light concentration) in terms of the structure.
On the other hand, not a hologram diffraction pattern laser-imprinted on a photosensitive material but a CGH: Computer Generated Hologram formed by calculating a diffraction pattern by a computer is also proposed.
Patent Document 2 discloses an optical splitter having at least one first optical system, a hologram element having a diffraction grating pattern for splitting light emitted radially from the first optical system into a plurality of luminous fluxes, and focusing the luminous fluxes to a plurality of spots, and a plurality of second optical systems with the light-receptive parts disposed at positions corresponding to the plurality of spots. It is described that determination of the optical phase of the diffraction plane of the hologram element can be designed by a computer generated hologram technology using a computer. However, such a conventional hologram element cannot concentrate incident light in a wide angular range.
Incidentally, Patent Document 3 discloses that the computer generated hologram is calculated using Fourier transform. However, for the computer generated hologram disclosed in Patent Document 3, the base material is quartz, and the resulting gradation pattern also remains on the quartz surface. For this reason, the uses thereof are limited to display devices and the like, and the computer generated hologram is not used for light concentration.    [Patent Document 1] JP-B 3687836    [Patent Document 2] JP-A 2002-228819    [Patent Document 3] JP-A 2004-206254    [Non-Patent Document 1] Glenn Rosenberg, et al., “Holographic planar concentrator increases solar-panel efficiency”, Laser Focus World Japan, Japan, 2009, vol. 2, p. 26-28